This invention relates to mudline suspension systems such as typically are used in drilling, temporary abandonment and completion operations conducted from a jack-up rig or other bottom supported platform in offshore oil and gas production. More particularly, the invention provides a means for adapting such systems for connection to a subsea wellhead, without the use of tieback hangers.
In a mudline suspension system the casing and tubing strings are suspended at or near the mudline, from hangers respectively supported on landing shoulders each located near the top of the next outer casing string. In production mode, the hangers are usually tied back to a surface wellhead on the rig or production platform. Sometimes however (e.g. in satellite wells), a subsea wellhead may be employed. For this purpose, an adapter wellhead is secured to the upper end of the conductor casing. A rigidizing mechanism ensures that externally applied loads on the adapter wellhead are transferred to the conductor casing. Hangers of the mudline suspension system are tied back to further hangers and packoffs received in the adapter wellhead. The adjustment required to tie back the mudline suspension system to the adapter wellhead is provided by adjustment subs accommodated in the adapter wellhead/conductor casing. A subsea Christmas tree is then secured and sealed to the adapter wellhead.
The resulting large stackup height can give difficulties in providing trawl protection. Also, duplication of hangers and provision of adjustment subs leads to a relatively large and complex assembly. Present designs of subsea completions are intended primarily for deeper water vessels, rather than for jack up deployment in shallower waters. Although such completions, including the required adapter wellheads, can be deployed from jack-ups, their bulk and weight lead to handling difficulties and increased capital/operational costs.
In accordance with the present invention there is provided a subsea Christmas tree coupling for mounting upon a mudline suspension system, having a radially outer connection adapted to make a rigid joint with a conductor casing and a radially inner connection adapted to make a sealed joint directly with a casing hanger assembly of the mudline suspension system without the use of a tieback hanger. The invention thus substantially eliminates the use of additional hangers besides those of the mudline suspension system, simplifying the coupling assembly, and providing the possibility of reduced stackup heights. The relatively compact and simple coupling thus obtained gives shorter installation times and lower costs. The rigid joint between the coupling and the conductor casing ensures that any external loading is transferred to the conductor casing.
Preferably the radially outer connection is made in use with a conductor housing attached to the conductor casing and in which the coupling is rigidly received.
The coupling may be an integral part of a subsea Christmas tree, or a separate tree may be mounted upon the coupling.
The tree may be either of horizontal or non-horizontal (i.e. conventional) configuration. In the case of a horizontal tree, the tubing is suspended from a hanger including a lateral production fluid outlet, landed in the tree, to permit the hanger and tubing to be pulled without disconnection of the tree. In the case of a conventional tree, the tubing may be suspended from a hanger received within the coupling, the radially inner connection being with a casing hanger of the mudline suspension system.
Further preferred features and advantages of the invention are in the following description of illustrative embodiments, made with reference to the drawings.